The geologic period with the most interesting story behind its naming, a rancorous academic turf battle.
The earliest attempts to pull a geologic time series out of rock layers (stratigraphy) were based upon comparing similar types of rocks in England and Scotland. Early British geologists hadn't even bothered to recognize differences in rocks below the Old Red Sandstone layer that lies under most of England, and reveals itself in a band from southwest Wales up into Scotland, and on the Shetland Islands. Between this layer, the bottom of the "Secondary" Era, and the crystalline rock layers of the "Primary" (Precambrian) Era lay a "transitional" layer of heavily folded graywackes, slates, and flagstones (The Transitional and Secondary Periods have since been reorganized into the Paleozoic Era).
Two people who decided to prize apart the transitional layer were Adam Sedgwick, a Cambridge University geology professor, and Roderick Impey Murchison, a wealthy Scotsman who happened to be interested in geology. Things went well at first. Sedgwick, working in north Wales (with his assistant Charles Darwin), and Murchison, working in south Wales, worked out an 1835 division of the transitional layer into two periods: the older Cambrian, after Cambridge University, and the later Silurian, after the Silures, a Celtic tribe of South Wales.
Here's where they ran into trouble: Murchison, studying rocks of the Silurian, had plenty of fossils with which to correlate his findings. He also had much more money, and was able to travel abroad to examine other rocks. Sedgwick, studying his Cambrian rocks in North Wales, had few fossils and had to rely on corellating rock types. It also appeared that the rocks that Sedgwick placed in the "Upper Cambrian" were rocks that Murchison was placing in the "Lower Silurian". (Murchison also claimed that graywackes of north Devon were part of the Silurian).
The two still got along fine. In 1839, they agreed that the Devon graywackes and the Old Red Sandstone were younger than Siluran age, defining a new "Devonian" period. However, the more Murchison and other investigators looked, the more inconsistencies appeared in Sedgwick's stratigraphy. Murchison began to state that the Cambrian was not a separate time period, and should be folded into the Silurian1.
Sedgwick's Cambrian system began to be dismantled piece by piece. It appeared to him that Murchison was taking all of the credit and all of the prestige which accompanied describing the rocks showing the beginning of life on Earth (which was thought at the time to have happened somewhere in the Cambrian-Silurian). In 1853, Sedgwick found a layer with Cambrian fossils above a layer claimed by Murchison to be of Silurian age! Each man was eager to extend the borders of, and defend every inch of, his geological fiefdom. Relations between the two became downright frosty, and there is little doubt that Royal Geological Society meetings became quite interesting.
The problem was, the usefulness of dating rocks by similar type alone is quite limited. In Wales, there is an unconformity on either side of the disputed rock layers. Not only that, rocks have been heavily folded by later mountain-building events, and some stratigraphic sequences have been turned upside-down. Later geologists relied much more on the more accurate technique of dating rocks by the fossils they contain.
In 1879, Scottish geologist John Lapworth resolved the issue. Showing that the disputed layers had distinctive graptolite and coral fossils, he placed them into their own time period. He chose the name "Ordivician" after the "Ordovices", a Celtic tribe that never submitted to Roman rule, and led a series of revolts against the Romans that led to their destruction in 78 AD. Although rocks of Ordivician age appear in the area once inhabited by the Ordovices, it has been said (and is much more fun to believe) that he chose the name of this warlike tribe to commemorate the controversy.
Roman-Britain: The Celtic Tribes of Britain: The Ordovices
http://www.roman-britain.org/tribes/ordovices.htm
"Geologic Time and the Cambrian Silurian Controversy of the Nineteenth Century."
Jim Talbot, Dept. of Geology, WWU
http://www.ac.wwu.edu/~talbot/wales/talk.htm
1Take a look at Webster 1913's definition of Cambrian and Silurian.